Hi, me again, and I'm sorry that I'm asking all this here :$

1) What's the difference between decals and overlays? Mainly: what's the impact on compiletime?

2) I'd like to create a river with quite a few turns, I'll be using displacement to dig out the river, but how would I add water? Just add a huge block of water that clips the edges of the displacement? Is it possible to put different brushes next to eachother (all with water texture applied)? (in hl1 mapping, you would notice a difference when using waves.

3) Who loved dod_schwetz?

my first experimental map had water and I used 1 brush with the water material applied :)

Yes you can use multiple brushes next to each other for water and can't see it in game.

I'm not entirely sure what the difference is but generally I find that overlays appear to be the right size wheras decals stretch to a huge size (how technical). Also overlays work on displacements, as they lay flat on a non-flat brush instead of floating on it (if that make's any sense) anddd you can apply overlays to more than 1 brush.

So.. overlays not decals :P

And I don't think they have a huge affect on compile time. The official maps I've decompiled seem to have hundreds of them.

Decals VS overlays:

Decals:

pros: very cheap to render, static therefore easy to compile

cons: can't be rotated, stretched

Overlays:

pros: can be rotated, stretched

cons: cost a lot more resource wise than decals (though we're talking peanuts if you don't plan on having hundreds visible at any one time), and when stretched or rotated, the compile tools need to apply that new texture over the new area, therefore they increase compile times.



As for your water, yes you should use multiple water brushes (always on the same level) and displacements for the riverbanks.

For an example, check this first screenshot of my new upcoming map: dod_rive:

http://perso.wanadoo.fr/froughol/907th/dod_rive/dod_rive0001.jpg

Though it's not very clear on the shot, you'll see that the riverbanks are heavily displaced. I used multiple brushes for the water, to fit a general sense of direction for the river, and many brushes to fit those water brushes, which I then converted to displacements (level 2) and applied the subdivide tool to give them a nice eroded effect. A good blend texture (in my case argentan's) with alpha blending will give you a nice mud effect for the river, merging into grass as you leave the water side.

Here's a couple of links that answer your questions:

Overlays (http://developer.valvesoftware.com/wiki/Hammer_Overlay_Tool)
(I found somewhere a decal vs overlay wiki page but lost it)

Water under displacements (http://www.student.kun.nl/rvanhoorn/Optimization.htm#chapter1)

Water is rendered anyways under displacements so the idea is to have as many water brushes as you like but only covering the area you can see.